1. Field of the Invention
The present invention relates to CMOS active pixel sensors. More particularly, the present invention relates to such pixel sensors having depleted photocollectors coupled to a depleted common node.
2. The Prior Art
Active pixel sensors that use a pinned photodiode are well known in the art. See, for example, U.S. Pat. No. 5,904,493. The pinned photodiode is advantageous as it prevents the high leakage current due to surface defects in the semiconductor materials used to make the photodiode. An additional improvement to the pinned photodiode is the fully depleted pinned photodiode as shown, for example, in U.S. Pat. No. 6,388,243. The depleted photodiode allows for reduction in photodiode capacitance which improves light sensitivity and cancellation of reset noise which improves signal-to-noise ratio, in addition to the lower leakage of the pinned diode. Most active pixel sensors that employ a pinned photodiode also have a sensing node coupled to the diode, usually through a transistor. The sensing node can also be pinned to improve performance by reducing capacitance and leakage effects as shown in U.S. Pat. No. 7,119,322. Another trend in active pixel sensor design is the reduction of pixel size and the desire to reduce the amount of circuitry within each pixel. Reducing the pixel size reduces the photocollection area, thereby reducing sensitivity. In order to counteract this sensitivity reduction, it is desirable to share readout circuits among more than one pixel as shown in U.S. Pat. No. 6,093,670. However, sharing the circuits can introduce problems such as increased capacitance and noise.
A new class of image sensors uses multiple detectors stacked in a vertical arrangement to make a multi-color sensor as exemplified in U.S. Pat. No. 6,727,521. The multi-color sensor that employs vertically stacked pixel sensors also benefits from sharing of wires and circuitry, particularly because the stacked pixel design has more photocollection sites per unit area of the sensor as shown in U.S. Pat. No. 6,960,757. Sharing the sense node of a single readout circuit among a number of photocollection circuits in a vertical stacked pixel sensor can also lead to degradation of performance similar to the conventional sensor, and therefore a way to counteract the negative effects of sharing readout circuits is needed. The invention embodied herein provides a method for improving the performance of an active pixel sensor that uses a shared readout node.